Code Division Multiple Access (CDMA) systems are spread spectrum systems in which different physical channels are separated by different spreading codes, the so-called channelization codes. Hence, the received signal is modulated by the data to be transmitted and by the channelization code. The data modulation is performed at a predetermined bit rate, and the modulation with the channelization code is performed at a higher rate, the so-called chip rate, as the bits of the spreading signal are referred to as chips. The spreading factor corresponds to the ratio of the data bit duration to the chip duration. Hence, the spreading factor determines the number of chips contained in one symbol.
In the wide-band code-division multiple access (WCDMA) system designed for 3GPP, the channelization codes are so-called orthogonal variable spreading factor (OVSF) codes. They are selected as to preserve orthogonality between different physical channels with different rates and spreading factors.
Furthermore, the downlink data in 3GPP WCDMA is further modulated according to a scrambling code, i.e. a spreading code specific for each cell, while the channelization code separates different physical channels within that cell. During an initial cell search procedure, the user terminal/receiver determines for all candidate scrambling codes whether there is significant signal energy correlated to a known channelization code. Hence, in a given cell, a CDMA receiver needs to determine the amount of signal energy correlated to one or more channelization codes of the received signal. Furthermore, the receiver needs to determine any frequency shifts and/or time delays of the received signal. Hence, the receiver performs a synchronisation in code space, in frequency space, and in time.
In order to perform the above synchronizations, a receiver in a WCDMA system needs to detect the presence of energy in a signal. In the 3GPP WCDMA, known pilot symbols are sent via a common pilot channel (CPICH) which is modulated with a known channelization code. Since the CPICH has a known channelization code and since the data sent via the CPICH is fixed and known to the receiver, the CPICH may be used in the cell search.
Similarly, in the path search and RAKE delay procedure, the synchronisation of the frequency and/or time requires a detection of peaks in the signal energy for different time/frequency delays.
Hence, in the above situations, the receiver determines a detection signal indicative of the energy in a de-spread signal in order to perform signal synchronisation in code space, frequency, and/or time. It is thus a general desire in WCDMA systems to increase the detection performance when determining the detection signal.
U.S. Pat. No. 5,691,974 describes a method for tracking the frequency and phase of a user channel in a spread spectrum system. According to this method, multiple spread spectrum communications signals are fed in parallel to respective data receivers where they are de-spread using respective preselected de-spreading codes at an adjustable phase angle and decoded over multiple orthogonal codes active within the communication system. Multiple decoded signals are then combined to form a single phase detection signal for use in a tracking loop. Even though this method improves the detection performance by using correlation of different channels, it remains a problem to further improve the detection performance.